The present invention relates to process variable transmitters used in process control and monitoring systems. More specifically, the present invention relates to detecting process noise frequency and adjusting sensor control signals so the sensor control signal frequency is moved away from the detected noise frequency to avoid error induced by the process noise.
Process variable transmitters are used to measure process parameters in a process control or monitoring system. Microprocessor-based transmitters often include a sensor, an analog-to-digital converter for converting an output from the sensor into a digital form, a microprocessor for compensating the digitized output, and an output circuit for transmitting the compensated output. Currently, this transmission is normally done over a process control loop, such as a 4-20 mA control loop, or wirelessly.
One exemplary parameter that is measured by such a system is the volumetric flow rate of a conductive fluid through a tube or pipe in the process. This can be done, for example, using a magnetic flow meter.
One exemplary type of magnetic flow meter includes a pipe, with coils disposed on opposite sides of a cross section of the pipe. The coils are energized by a coil drive current from a transmitter and develop a magnetic field along the cross section of the pipe. Two electrodes are located across the pipe from each other along a line which is perpendicular to the magnetic field. Fluid passing through the pipe is electrically conductive. As a result of the movement of the conductive fluid through the magnetic field, an electric potential or electromotive force (EMF) is induced in the fluid which is detected by the electrodes.
These types of magnetic flow meters are often deployed in processes that have various substances added to the flow of fluid through the pipe, that is being sensed and measured. For instance, in some paper processing mills, silica is added to the fluid flowing through the pipe in the paper process. The silica can be added to the process using a piston pump that is controlled by a variable frequency drive motor to control the flow of the silica added to the process.
Piston pumps normally operate using a set of pistons that are alternatively connected to an input port and an output port. The substance to be pumped is alternately drawn in through the inlet port and then pumped out through the outlet port by the reciprocating action of the piston. When this type of pump is used to add a substance to fluid flowing through a pipe in a process, the pump can add a pressure modulation to the flow that corresponds to the speed of operation of the pump. This pressure modulation can be observed by the magnetic flow meter and can contain significant energy at the odd harmonics of the coil drive frequency used to drive the coils in the magnetic flow meter. This can result in a beat frequency to appear on the measured output signal that measures the process flow.
Of course, this type of introduction of periodic noise caused by a piston pump (which interferes with the measurement of flow) is but one exemplary source of periodic or intermittent noise that can interfere with the measured output signals that represent measured process variables. A variety of other noise sources can create noise that interferes with the measurement of process variables in a similar way. For instance, similar types of noise can be generated by sources that cause pulsating pressure, vibration, and external electrical interference, to name a few.